1. Field of the Invention
The field of the invention is that of reflector antenna feed devices. The invention also relates to an antenna equipped with this kind of feed.
2. Description of the Prior Art
Patent application EP 1 221 740 describes an antenna 1 having a main reflector 10 and a feed 12, with reference to FIG. 1 of that application, which is reproduced as FIG. 1 of the present application. The antenna 1 features symmetry of revolution about an axis OO′ of the antenna. FIG. 1 represents a diagrammatic half-section in a plane containing the axis OO′ of symmetry. The antenna 1 comprises a main reflector 10 having a concave side in the shape of a paraboloid rotated about the axis OO′, for example, so that it is markedly directional in the direction of the axis OO′. An antenna feed device 12 is situated along the axis OO′ of the antenna 1, in the portion of the reflector comprising the concavity. Like the whole of the antenna, it has symmetry of revolution about the axis OO′. The feed device 12 is shown in more detail in FIG. 2. It comprises a waveguide portion 20 extending along the axis OO′, in the direction from the center of the reflector 10, and within the concavity. With regard to the feed 12, it is considered that a first end 21 of the waveguide 20 consists of the location at which the waveguide 20 passes through the main reflector 10. This first end is situated at the center of the main reflector 10. A second end 22 of the waveguide 20 faces a subreflector 24. The subreflector 24 intersects the axis OO′. It has a shape obtained by rotation about the axis OO′. It has a convexity that faces toward the concavity of the main reflector 10. The outside diameter of the subreflector 24 is greater than the diameter of the waveguide 20. The exact shape of this subreflector 24 is defined by its function. In a receiving mode, the subreflector 24 reflects the electromagnetic waves coming from the main reflector 10 toward the waveguide 20. In a sending mode, the subreflector 24 reflects the electromagnetic waves coming from the waveguide 20 toward the reflector 10. In order to confine the electromagnetic waves between the second end 22 of the waveguide 20 and the subreflector 24, a portion of the feed 12 consists of a dielectric body 23 joining the second end 22 of the waveguide 20 and the subreflector 24. The confinement of the electromagnetic waves between the second end 22 of the waveguide 20 and the subreflector 24 improves the electromagnetic coupling between the subreflector 24 and the main reflector 10.
The dielectric body 23 has a portion 31 outside the waveguide 20 and a portion 30 inside the waveguide. Because of the difference between the diameter of the subreflector 24 and the diameter of the waveguide 20, an exterior surface 29 of the dielectric body 23 has a frustoconical shape with two ends, one of small diameter and the other of large diameter. The small diameter end is connected to the second end 22 of the waveguide 20. The small diameter is substantially equal to the diameter of the waveguide 20. The large diameter is substantially equal to the outside diameter of the subreflector 24.
To improve the coupling between the dielectric body 23 and the air around the frustoconical surface 29 of the dielectric body 23, the latter is provided with grooves or creases having symmetry of revolution about the axis OO′. Because of this, the frustoconical surface 29 has bosses 25 and hollows 28. These creases prevent the electromagnetic waves propagating along the surface of the subreflector 22, whether the electric field of these waves is normal or tangential to that surface. As a result of this the directional diagram of the antenna 1 is more directional in the direction of a main lobe of the antenna, and there is therefore lower dispersion in the secondary lobes. The subreflector 24 generally consists of a metallic deposit on a surface of the dielectric body 23. The concave volume delimited by the metallic deposit constituting the subreflector 24 is generally filled with a dielectric. The portion 30 of the dielectric body inside the waveguide has at the end 22 a portion 27 whose diameter is equal to the inside diameter of the waveguide 20. This portion 27 is extended in the direction of the first end 21 by a second portion 26 whose diameter decreases at one step or at several successive steps. This structural feature improves the electromagnetic coupling between the waveguide 20 and the dielectric body 23. This reduces reflection losses in particular.
Although the antenna that has just been described has improved qualities compared to other antennas that do not have these features, this antenna has a bandwidth that is limited by the constraints imposed on the maximum permitted value of the reflection loss rate. Its radiation diagram has a directional gain that is limited because of a lack of phase efficiency and thus relatively high levels in the secondary lobes. Remember that the phase center is defined as the center of a spherical wavefront. In the ideal case, that center is a point, in which case the phase efficiency is equal to 1. In practice, the center is not clearly defined and is more like a small volume. In this case the phase efficiency is less than 1. The phase efficiency of a radiation diagram may be calculated from the following formula PE1;                               Phase          ⁢                                          ⁢          efficiency                =                                            |                                                ∫                                      θ                    1                                                        θ                    2                                                  ⁢                                                                            cos                      45                                        ⁡                                          (                      θ                      )                                                        ⁢                  tan                  ⁢                                      θ                    2                                    ⁢                                      ⅆ                    θ                                                              ⁢                              |                2                                                                    [                                                      ∫                                          θ                      1                                                              θ                      2                                                        ⁢                                      |                                                                  cos                        45                                            ⁡                                              (                        θ                        )                                                              |                                          tan                      ⁢                                              θ                        2                                            ⁢                                              ⅆ                        θ                                                                                            ]                            2                                ≈                                    |                                                ∑                                      n                    ⁢                                                                                  ⁢                    θ                                                  ⁢                                                                            cos                      45                                        ⁡                                          (                      θ                      )                                                        ⁢                  tan                  ⁢                                      θ                    2                                                              ⁢                              |                2                                                                    [                                                      ∑                                          n                      ⁢                                                                                          ⁢                      θ                                                        ⁢                                      |                                                                  cos                        45                                            ⁡                                              (                        θ                        )                                                              |                                          tan                      ⁢                                              θ                        2                                                                                            ]                            2                                                          (        PE1        )            
In the above formula, COS45(θ) is the copolar component of the electric field in the plane at 45°.
Compared to the prior art just described, the present invention aims to improve further the coupling between the waveguide 20 and the main reflector 10, in particular by reducing the reflection loss rate. As a result, with the same constraints as in the prior art as to the maximum allowed value of the reflection rate, the bandwidth of an antenna using a feed according to the invention is increased. The invention also aims to improve the phase efficiency of the antenna, which has the effect of improving the radiation diagram of the antenna so that a greater proportion of the total energy broadcast is in its main lobe. Finally, the invention aims to simplify the shape of the dielectric body and thus make it simpler to fabricate.
Finally, using the invention means that, for the same antenna efficiency, it is possible to retain a small subreflector produced by a metallic deposit on a rear face of the dielectric.